This invention relates, in general, to pick up tools, and, more particularly, to positive stop, pick up tools for the accurate placement of integrated circuit (IC) dice on a packaging substrate.
This type of tool is designed for use in placing an IC die on a die attach material disposed on the surface of the packaging substrate. Presently, a resistance sensing pick up is utilized that measures the amount of resistive force bond being exerted on the die from the die attach material. Various problems result from this type of IC die placement.
One of the problems results from environmental changes or varying die attach material mixtures. These types of changes can cause the viscosity of the die attach material to vary which will cause the amount of resistance, encountered by the pick up when placing the die on the substrate, to vary. This causes the die to be placed at different heights above the substrate causing different thicknesses of die attach material (epoxy) to be left between the substrate and the die.
By way of example, one particular die attach material, silver glass paste, requires that, when cured, a minimum height of 0.002 inches of dry die attach material or the die attach material will not reliably hold the die to the substrate. In order to have 0.002 inches of die attach material dry, a wet die attach material thickness of 0.004 inches must be maintained. If the viscosity of the die attach material is reduced from the normal, due to environmental changes or die attach material composition, the resistance encountered by the pick up will be reduced causing the die to be placed closer to the substrate. This then causes the thickness of the die attach material to be less than required.
In addition, problems can be caused by the allowable tolerances in the thickness of the packaging substrate. In adding up the tolerances of the various layers of the substrate it is not unusual for a total thickness tolerance of .+-.0.004 inches to be present. Using the type of die attach material agent referred to above, requiring a thickness of 0.004 inches wet, this tolerance can result in the reduction of the thickness of the die attach material to a level below the required minimum.
A further problem results from substrates that do not have level surfaces across the plane where the die is to be attached. Since the present pick up tools only measure the overall resistance exerted by the die attach material, if one corner of the substrate area is higher, due to allowable package fabrication processing tolerances, than the other three corners, then the corresponding die corner will be closer to the substrate than the remaining three corners. This causes a die attach material thickness of less than 0.002 inches for a portion of the die attach material which will not provide the required bonding.